Hello Richard, while planing a multiday trip, I noticed that gradient display (in the 2d view) breaks.
for instance on this route: https://cycle.travel/map/journey/306662 around kilometer 190, it doesn't show any color, and the tooltip on the line of the 2D view doesn't show the percentage like it usually does (can't screenshot the tooltip as it disapears). however this is actually a very steep climb albeit for my touring standards.
Now if I draw only that segment of the road in a new tab, it show correctly ( https://cycle.travel/map/journey/316733 ):
while experimenting I've noticed the tooltips start to break at around 150km as for the coloring, I haven't really found a logic as to when it breaks. Also note that zooming in on the elevation chart doesn't help.
hope this helps!